Principles Phonon Calculations Research Articles

Polar GaN Surfaces under Gallium Rich Conditions: Revised Thermodynamic Insights from Ab Initio Calculations.

This paper presents an improved theoretical view of ab initio thermodynamics for polar GaN surfaces under gallium-rich conditions. The study uses density functional theory (DFT) calculations to systematically investigate the adsorption of gallium atoms on GaN polar surfaces, starting from the clean surface and progressing to the metallic multilayer. First principles phonon calculations are performed to determine vibrational free energies. Changes in the chemical potential of gallium adatoms are determined as a function of temperature and surface coverage. Three distinct ranges of Ga coverage with very low, medium, and high chemical potential are observed on the GaN(000-1) surface, while only two ranges with medium and high chemical potential are observed on the GaN(000-1) surface. The analysis confirms that a monolayer of Ga adatoms on the GaN(000-1) surface is highly stable over a wide range of temperatures. For a second adlayer at higher temperatures, it is energetically more favorable to form liquid droplets than a uniform crystalline adlayer. The second Ga layer on the GaN(0001) surface shows pseudo-crystalline properties even at a relatively high temperature. These results provide a better thermodynamic description of the surface state under conditions typical for molecular beam epitaxy and offer an interpretation of the observed growth window.

Dynamic Lone Pair Expression as Chemical Bonding Origin of Giant Phonon Anharmonicity in Thermoelectric InTe

AbstractLoosely bonded (“rattling”) atoms withs2lone pair electrons are usually associated with strong anharmonicity and unexpectedly low thermal conductivity, yet their detailed correlation remains largely unknown. Here we resolve this correlation in thermoelectric InTe by combining chemical bonding analysis, inelastic X‐ray and neutron scattering, and first principles phonon calculations. We successfully probe soft low‐lying transverse phonons dominated by large In1+z‐axis motions, and their giant anharmonicity. We show that the highly anharmonic phonons arise from the dynamic lone pair expression with unstable occupied antibonding states induced by the covalency between delocalized In1+5s2lone pair electrons and Te 5pstates. This work pinpoints the microscopic origin of strong anharmonicity driven by rattling atoms with stereochemical lone pair activity, important for designing efficient materials for thermoelectric energy conversion.

Dynamic Lone Pair Expression as Chemical Bonding Origin of Giant Phonon Anharmonicity in Thermoelectric InTe.

Loosely bonded ("rattling") atoms with s2 lone pair electrons are usually associated with strong anharmonicity and unexpectedly low thermal conductivity, yet their detailed correlation remains largely unknown. Here we resolve this correlation in thermoelectric InTe by combining chemical bonding analysis, inelastic X-ray and neutron scattering, and first principles phonon calculations. We successfully probe soft low-lying transverse phonons dominated by large In1+ z-axis motions, and their giant anharmonicity. We show that the highly anharmonic phonons arise from the dynamic lone pair expression with unstable occupied antibonding states induced by the covalency between delocalized In1+ 5s2 lone pair electrons and Te 5p states. This work pinpoints the microscopic origin of strong anharmonicity driven by rattling atoms with stereochemical lone pair activity, important for designing efficient materials for thermoelectric energy conversion.

Thermodynamics for the non-conventional synthesizing of out-of-plane ordered double-transition metal “312” and “413” MAX phases (o-MAX): A high throughput linear programing first-principles calculation

• Temperature corrections at the typical sintering temperature were addressed properly. • A strong correlation between actual formability of o-MAX phases and the DOS at the Fermi level were found. • Using an updated linear programing optimization algorithm, results of the preparation of o-MAX phase in the existing experiments can be predicted well. • New non-conventional synthetic routes were suggested, expected to improve purity and yield. The reaction thermodynamics for synthesizing the “312” and “413” o-MAX phases using the powder metallurgy are investigated using a linear programing optimization algorithm based on the high-throughput first principles phonon calculations. The validity and reliability of the current methodology are verified by correctly predicting the impurities in four experimentally known o-MAX systems including Cr-Ti-Al-C, Cr-V-Al-C, Mo-Sc-Al-C and Mo-Ti-Al-C. The formability of each investigated o-MAX phase is evaluated by means of formation enthalpy and formation Gibbs free energy in a temperature range from 0 K to 1700 K. It is revealed that the thermodynamic stability of the “413” o-MAX structure is no better than that of the “312” phase. The formability of “413” o-MAX is also reduced at high sintering temperature, compared to that of “312” phase. The optimal synthetic routes are predicted for all thermodynamically stable “312” and “413” o-MAX phases. It is found that most o-MAX phases considered could be prepared as the single phase using the non-conventional synthetic routes from the aspect of reaction thermodynamics. Few of them including Cr 2 TaAlC 2 , Nb 2 HfAlC 2 , Nb 2 TaAlC 2 , Nb 2 Hf 2 AlC 3 , Nb 2 Ta 2 AlC 3 , Mo 2 V 2 AlC 3 and Mo 2 Ta 2 AlC 3 are predicted to be either destabilized at high temperature or overwhelmed by the most competing side reaction.

Collective Atomic Motion in Crystals Under Shear Stress by First Principles Phonon Calculations

Collective Atomic Motion in Crystals Under Shear Stress by First Principles Phonon Calculations

Combined experimental and ab initio based determination of the thermal expansion of La0.5Sr0.5Co0.25Fe0.75O3

AbstractThe thermal expansion of La0.5Sr0.5Co0.25Fe0.75O3 (LSCF55) is investigated both by first principles phonon calculations combined with the quasi‐harmonic approximation (QHA) and by experimental approaches. Within the framework of the QHA, the volumetric thermal expansion coefficient of rhombohedral LSCF55 is calculated as αV,GGA = 50.34 × 10−6 K−1. For comparison, the lattice expansion and the volume expansion of LSCF55 grain are measured by in situ high‐temperature X‐ray diffractometer (HT‐XRD). An anisotropic thermal expansion of rhombohedral LSCF55 with αa,hex = 10.89 × 10−6 K−1 and αc,hex = 21.18 × 10−6 K−1 is obtained. The volumetric thermal expansion coefficient is measured as αV,HT‐XRD = 43.17 × 10−6 K−1. In addition, the effectively isotropic expansion coefficients of a polycrystalline LSCF55 bar specimen are measured using a vertical high‐performance thermo‐mechanical analyzer and yield αl,bar specimen = 17.37 × 10−6 K−1 and αV,bar specimen = 52.11 × 10−6 K−1.

Thermochemical properties of rare-earth oxyhydrides from first principles phonon calculations

We determine the thermochemistry of the series of rare-earth oxyhydrides to predict possible synthesis routes and decomposition mechanisms.

On the relationship between chemical expansion and hydration thermodynamics of proton conducting perovskites

This contribution explores the relationship between, and the compositional dependence of, the chemical expansion and entropy of hydration of the proton conducting perovskites BaZrO3, BaSnO3, BaCeO3, and SrZrO3 by first principles phonon calculations.

Anisotropy in lattice thermal conductivity tensor of bulk hexagonal-MT2 (M = W, Mo and T = S and Se) by first principles phonon calculations

Anisotropies in phonon mean free path and thermal conductivity as a function of temperature are calculated for 2H-MT2 structures.

Unravelling the fundamentals of thermal and chemical expansion of BaCeO3 from first principles phonon calculations.

Differentiating chemical and thermal expansion is virtually impossible to achieve experimentally. While thermal expansion stems from a softening of the phonon spectra, chemical expansion depends on the chemical composition of the material. In the present contribution, we, for the first time, completely decouple thermal and chemical expansion through first principles phonon calculations on BaCeO3, providing new fundamental insights to lattice expansion. We assess the influence of defects on thermal expansion, and how this in turn affects the interpretation of chemical expansion and defect thermodynamics. The calculations reveal that the linear thermal expansion coefficient is lowered by the introduction of oxygen vacancies being 10.6 × 10-6 K-1 at 300 K relative to 12.2 × 10-6 K-1 for both the protonated and defect-free bulk lattice. We further demonstrate that the chemical expansion coefficient upon hydration varies with temperature, ranging from 0.070 to 0.115 per mole oxygen vacancy. Ultimately, we find that, due to differences in the thermal expansion coefficients under dry and wet conditions, the chemical expansion coefficients determined experimentally are grossly underestimated - around 55% lower in the case of 10 mol% acceptor doped BaCeO3. Lastly, we evaluate the effect of these volume changes on the vibrational thermodynamics.

First principles phonon calculations in materials science

Phonon plays essential roles in dynamical behaviors and thermal properties, which are central topics in fundamental issues of materials science. The importance of first principles phonon calculations cannot be overly emphasized. Phonopy is an open source code for such calculations launched by the present authors, which has been world-widely used. Here we demonstrate phonon properties with fundamental equations and show examples how the phonon calculations are applied in materials science.

Thermodynamic properties of neutral and charged oxygen vacancies in BaZrO3 based on first principles phonon calculations.

The structural, electronic and thermodynamic properties of neutral and positively doubly charged oxygen vacancies in BaZrO3 are addressed by first principles phonon calculations. The calculations are performed using two complementary first principles approaches and functionals; the linear combination of atomic orbitals (LCAO) within the hybrid Hartree-Fock and density functional theory formalism (HF-DFT), and the projector augmented plane wave approach (PAW) within DFT. Phonons are shown to contribute significantly to the formation energy of the charged oxygen vacancy at high temperatures (∼1 eV at 1000 K), due to both its large distortion of the local structure, and its large negative formation volume. For the neutral vacancy, the resulting lattice distortions, and thus the contributions from phonons to the free formation energy, are significantly smaller. As a result, phonons affect the relative stability of the two defects at finite temperatures and the charge transition level for oxygen vacancies (+2/0) changes from 0.42 to 0.83 eV below the conduction band bottom from 0 K to 1000 K.

First Principles Calculations of Advanced Nitrides, Oxides and Alloys

The phase stability of silicon nitride is examined using a series of first principles phonon calculations. -phase shows slightly higher free energy than  in the temperature range from 0 to 2000K. The difference between  and is only 0.02 eV/Si3N4 or 2 kJ/mol at 300 K. The result is consistent with the experimental report by high-temperature oxide-melt-drop solution-calorimetry. Similar calculations are made for ZrO2, Ga2O3, BN and some perovskites. Using cluster expansion technique combined with a large set of first principles calculations, the cation disordering of II-III spinel oxides and the phase diagram of diamond vs. c-BN are theoretically investigated. Theoretical results on the atomic structures of  and  sialons are also shown.

Vibrational properties of Si/GaAs superlattices

Raman scattering experiments and first principles phonon calculations have been performed on (Si) m (GaAs) n superlattices grown by molecular beam epitaxy. In spite of the small thickness of the Si layers, folded acoustic modes, confined Si-like and quasi-confined GaAs-like optical modes are clearly observed in the spectra. The experimental frequencies compare well with the calculated ones, confirming that a description of optical phonons in terms of strain- and confinement-induced shifts is appropriate for these novel heterostructures.

Ideal and relaxed surfaces of SiC

During the past few years there has been considerable interest in the relaxation of GaAs(110) and Si(100)-(2×1) surfaces. The surfaces of SiC, however, provide an intermediate system between these heteropolar and homopolar structures. It is interesting therefore to investigate the kinds of relaxation that might occur at the zincblende and wurtzite surfaces of SiC. We perform calculations using Chadi’s energy minimization scheme, with the coefficients of linear, quadratic, and cubic energy correction terms fitted to the bulk lattice constant, bulk modulus, and thermal expansion coefficient. To check the validity of this model, we calculate six experimentally known phonon frequencies. The agreement between theory and measured values is quite good. With this model, we investigate the relaxation at the SiC(110) surface in the zincblende structure and SiC(101̄0) and (112̄0) surfaces in the wurtzite structure. The results show a combination of downward movement and buckling for all three surfaces. The reduction in total energy is about 0.21 eV/atom for the (110) surface, and 0.24 eV/atom, 0.18 eV/atom for (101̄0) and (112̄0), respectively. Further results include the determination of ideal and relaxed electronic structure and optimum relaxed geometries. Finally, a new theory which extends Chadi’s scheme to do first principles phonon calculations is presented and discussed.